十万个为什么·科学启蒙30篇（3）
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Earth’s Axis Points Toward One Star
地球轴指向一颗星，所以北极星几乎不动
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Earth spins like a top, and its axis points steadily toward a spot near Polaris.地球像陀螺一样自转，其地轴始终指向北极星附近。
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That means Polaris stays almost fixed while other stars seem to circle around it.因此，北极星几乎静止不动，而其他恒星看起来绕着它旋转。
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From the North Pole, it appears directly overhead; from the equator, it sits on the horizon.在北极点，它位于天顶正上方；在赤道，它则位于地平线上。
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Its position changes only very slowly over centuries due to wobble in Earth’s spin.由于地球自转轴的微小摆动，它的位置每几个世纪才发生极缓慢的变化。
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Other stars rise and set each night because Earth rotates under them constantly.其他恒星每晚东升西落，是因为地球持续自转，从它们下方经过。
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But Polaris stays put since it lines up with our planet’s turning center.但北极星保持不动，因为它恰好位于地球自转轴的延长线上。
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You can use it to find north when hiking or sailing at night.夜间徒步或航海时，可用它来辨别正北方向。
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Ancient sailors and explorers relied on it before GPS or compasses existed.在GPS和指南针出现之前，古代航海家和探险者一直依赖它导航。
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If you watch the sky for an hour, you’ll see stars swirling but Polaris staying still.若连续观测星空一小时，会看到群星旋转，而北极星岿然不动。
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It’s not the brightest star, but it’s the most useful one for finding direction.它并非最亮的恒星，却是辨识方向最有用的一颗。